1. Field of the Invention
The present invention relates to a battery charger termination circuit and more particularly to a battery charger termination circuit that functions in conjunction with known so called battery safety circuits which terminate battery charging after predetermined time periods under predetermined conditions to prevent battery cell damage resulting from extended battery charging cycles.
2. Description of the Prior Art
Various portable devices and appliances, such as cellular phones, require rechargeable batteries. Various types of rechargeable batteries are known to be used in such applications. For example, nickel-cadmium (NiCd), nickel metal hydride (NiMH), as well as lithium ion batteries are known to be used. Because of the different charging characteristics of such batteries, different battery chargers are required. For example, both nickel-cadmium (NiCd), as well as nickel metal hydride (NiMH) batteries, require constant current charging. On the other hand, lithium ion batteries require constant current charging up to a certain voltage value and constant voltage charging thereafter. Because of the different charging characteristics of the various battery types, different charging circuits are required. Examples of charging circuits for different battery types are disclosed in commonly-owned U.S. Pat. Nos. 5,764,030; 5,998,966 and 6,002,237, hereby incorporated by reference.
Various problems are known which can result in battery damage during battery charging. One problem is known as gassing. Gassing is a condition that occurs when a battery is charged below room temperature. More particularly, gassing relates to a build up of oxygen resulting from the chemical reaction that occurs within a battery cell during charging. When a battery is being charged below room temperature, the oxygen pressure can increase within the battery housing and exceed the limits of the housing thus causing damage to the battery cell. However, at higher temperatures, the oxygen recombines, thus reducing the risk of excessive pressure within the battery housing.
Various battery circuits have been developed to address this problem. For example, U.S. Pat. No. 4,667,143 discloses a battery charger which includes a thermistor for sensing battery temperature. The thermistor is connected in parallel with a temperature stable resistor to provide a temperature compensated battery voltage signal. The temperature compensated voltage signal is used to control the charging of the battery as a function of temperature.
Commercially available monolithic battery circuits are also known which control battery charging as a function of the battery temperature. For example, a Texas Instrument Model No. bq2057 advanced lithium ion linear charge management (IC) integrated circuit is available. This IC requires an external thermistor and is used to inhibit battery charging until the temperature of the battery is within user-defined thresholds. Thus the safety circuit can prevent charging of the battery when the temperature of the battery is below a predetermined threshold in order to prevent gassing.
Another known problem that occurs during certain charging conditions relates to overheating. Overheating occurs as a result of prolonged charging of a battery causing the temperature of the battery to increase to an unacceptable level, possibly causing damage. In order to address this problem, various battery charger circuits have been developed which limit charging times in order to reduce the possibility of overheating of the battery cell. For example, U.S. Pat. No. 4,035,709 discloses a battery charging circuit which monitors the battery voltage and terminates fast charging when the battery voltage reaches a predetermined level, for example, 80% of the desired voltage level. Once the battery reaches 80% of the desired voltage, rapid charging is terminated and a timer is enabled which allows trickle charging for a fixed period of time, for example, six hours. U.S. Pat. No. 5,727,232 also relates to a battery charger circuit which uses predetermined time periods to control charging cycles.
Due to the differences in battery charger circuits, battery manufacturers are known to incorporate battery safety circuits directly into the battery cell packages. More particularly, such battery cell packages are known to include one or more battery cells, serially connected to one or more switching devices, such as field effect transistors (FETs), which in turn, are serially coupled between the battery cell and the power supply terminals in order to interrupt charging of the battery under certain operating conditions. The FETs are under the control of a so-called battery safety circuit, usually an integrated circuit (IC), also disposed within the battery cell package. Such safety circuits are known to provide overcharge, over discharge and overcurrent protection.
Overcharge protection relates to a condition when a relatively large voltage is impressed upon the battery cell for an excessive time period. Over-discharge protection relates to a condition when the discharge current from the battery is excessive resulting in the battery voltage dropping below a predetermined voltage during a discharge mode (i.e. charger off mode). The overcurrent mode relates to a condition when the discharge current from batteries exceeds a level, indicative of a short circuit. The so called battery safety circuits monitor the discharge current and voltage in order to protect against the various conditions mentioned above. When one of the conditions, such as overcharge, over-discharge or overcurrent is sensed by the safety circuit, the safety circuit interrupts battery charging from within the battery cell, independent of the battery charger, by turning off one or more of the serially coupled FETs.
Various battery safety circuits are known. Examples of such battery safety circuits are available from Mitsumi Corporation. For example, Mitsumi Model No. MM1412 and MM1491 are battery safety circuits for use with lithium ion batteries. These devices are described in data sheets entitled: xe2x80x9cProtection of Lithium-Ion Batteries MM1412 xe2x80x9d and xe2x80x9cLithium-Ion Battery Protection (for 1-cell in series) MM1491xe2x80x9d, published by Mitsumi Corporation, hereby incorporated by reference.
Use of the battery safety circuits inside of the battery cell packages thus insures a certain level of battery protection irrespective and independent of the battery charger used to charge the battery cell packages. Although the battery charger circuits and circuits discussed above provide adequate protection for the various operating conditions, those circuits do not address battery charging conditions which can result in reduced battery cell life. In particular, battery cells are known to loose capacity if a maintenance of trickle charge is continued for hours or even days following a full charge. As such, appliances which utilize, for example, lithium-ion batteries which have a reduces life are more expensive to use. Thus there is a need for a battery circuit to protect against battery charging conditions that are detrimental to the battery cell life.
Briefly, the present invention relates to a charge termination circuit which may be disposed within the battery cell pack and used in conjunction with known battery safety circuits to protect against charging conditions which reduce the life of a battery cell. In particular, the charge termination circuit is adopted to be used in conjunction with an off the shelf battery safety, safety circuit implies the FET circuit which includes one or more power FETs coupled between the battery cell and the battery charger terminals on the battery cell package. The charge termination circuit includes a microcontroller and monitors the battery cell voltage and current by way of an I/O port configured as an A/D input and activates a first timer anytime the charging current to the battery is below a predefined threshold, for example, indicative of a maintenance or trickle charge. The charging current to the battery is measured across one or more of the FETs in order to avoid introducing an additional series impedance. When the first timer times out after a predetermined time period, for example, two hours, the charge termination circuit forces the one or more of the serially coupled FETs switches to interrupt battery charging of the battery cells. The charge termination circuit also monitors the battery cell voltage and closes the FETs to allow charging when the cell voltage falls below a predetermined level. As a second level of protection, each time the microcontroller is powered up, a second timer is activated. The second timer is set for a much longer period of time than the first timer, for example, six hours, and is used in the event that the charging current does not fall below the threshold discussed above. The second timer is used to terminate charging by switching one of the serially coupled power FETs off when the second timer times out. The charge termination circuit is adapted to work in conjunction with conventional battery safety circuit and provide and additional level of functionality with a battery cell package in order to provide protection against operating conditions which reduce battery cell life, heretofore unknown.